You say that you can change the language of your sub sites. Hm, ones you have choosen a language of your site, it will only be in that language.
If you create a top site, you will have the same choice as in creating a sub site. After you have clicked on Create / Sites and workspaces, youwill find yourself on the page named New SharePoint Site. In the section at the bottom, Language, you may choose the language you would like the Site to have.
Does this information help you?
/Michael Jansson
HumanData, Sweden
Michael Jansson
SharePoint Consultant + Trainer